The present invention relates to wavefront splitting elements for EUV light and phase measuring apparatuses using the same, and is suitable for measuring a phase of an optical thin film put on a reflection surface and transmission surface of an optical system used for an exposure apparatus of a step and repeat method, a step and scan method, etc. for fabricating devices, e.g., semiconductor devices such as ICs, LSIs, etc., image pick-up devices such as CCDs, etc., and display devices such as liquid crystal panels, etc.
Along with the miniaturization of device patterns, an exposure wavelength for projecting and exposing a device pattern onto a photosensitive material is becoming shorter and shorter. For example, an exposure wavelength is in transition from KrF (with a wavelength of 248 nm) to ArF (with a wavelength of 193 nm) and F2 laser (with a wavelength of 157 nm), and even EUV light with a wavelength of 13.4 nm has also reduced to practice.
A smaller device pattern is the most important factor that supports the dynamics in the semiconductor industry, and thus the age requiring for the resolution with a critical dimension (“CD”) of 250 nm for DRAMs is rapidly changing to that of CDs of 180 nm, 130 nm, and even 100 nm. The lithography using i-line (a wavelength of 356 nm) as exposure light has never required the resolution with a CD less than a wavelength.
On the other hand, KrF with a wavelength of 248 nm has been applied for the lithography that requires a CD of 180 nm or even 150 nm. The resolution with a CD less than a wavelength is about to reduce to practice by exploiting achievements in improved resists and the super-resolution techniques, etc. Various super-resolution techniques would possibly realize the pattern resolution with a CD of a ½ wavelength in the line and space.
However, the super-resolution techniques often go with pattern manufacturing restrictions, and the most effective way of improving resolution is, after all, to make shorter a wavelength of exposure light and to make higher the NA of a projection optical system. This fact greatly motivates to seek for shorter wavelengths of exposure light, leading to a development of the EUV lithography that uses light with a wavelength of 10–15 nm as exposure light.
The EUV light as exposure light requires an all-mirror system, and it is difficult to control the characteristics of a reflection enhancement film to be applied for expected reflectance onto the mirror. The materials for the film have also many restrictions, and its fundamental structure is typically made of a film (i.e., a multilayer film) of Mo and Si alternate layer. Other multilayers include, for example, Be—Si and Rh—Si multilayers. Where one pair is a Mo and Si alternate layer, a film requires a multilayer of 40 pairs, and provides extremely drastic changes of optical characteristics.
Characteristically, a film for the EUV region requires control over its phase (or phase distribution) as well as its reflectance. When the EUV light reflects on the film, its phase changes. The phase distribution in the film distorts the wavefront that enters a mirror surface, and an offset in the cycle length of the film in the mirror surface would cause an aberration (i.e., a wavefront aberration). Therefore, it is desirable to measure the film's phase (or phase distribution) as soon as the film is put onto the mirror surface. In particular, it is desirable to measure angular characteristics when light is brought into the film at various angles.
An angle of light incident to an actual mirror is so small that most incident light rests in an angular distribution range from several degrees at a small position to about 20 degrees at a large position. When the phase distribution of a film applied to the mirror surface is measured with various incident angles, the measurement close to an angle close to the perpendicular incidence is difficult because the beams superimpose in the reflection system as a measurement system.
In particular, when an interferometer is used to detect phase information dependent on an incident angle of a film applied to a mirror surface, it has been very difficult to measure the film's phase information at an incident angle of 0° since there are no appropriate materials available for the EUV light region for splitting the incident light and reflected light that go reversely along the same optical path.
Therefore, a conventional measuring system has exclusively used PDIs, shearing interferometers, etc. that have no need of beam splitters for the EUV region.